Small, high power optical isolator

ABSTRACT

An optical isolator for use with high power, collimated laser radiation includes an input polarizing optical element, at least one Faraday optical element, at least two reflective optical elements for reflecting laser radiation to provide an even number of passes through said at least one Faraday optical element, at least one reciprocal polarization altering optical element, an output polarizing optical element, at least one light redirecting element for remotely dissipating isolated or lost laser radiation. The isolator also includes at least one magnetic structure capable of generating a uniform magnetic field within the Faraday optical element which is aligned to the path of the collimated laser radiation and a mechanical structure for holding said optical elements to provide thermal gradients that are aligned to the path of the collimated laser radiation and that provide thermal and mechanical isolation between the magnetic structure and the optical elements.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a continuation of U.S. patent Ser. No.16/723,191, filed Dec. 20, 2019 which in turn claims the filing benefitsof U.S. provisional application Ser. No. 62/782,494, filed Dec. 20,2018, both of which are incorporated herein by reference in theirentirety.

FIELD OF THE INVENTION

The present invention relates to small, high power polarizationinsensitive and polarization maintaining optical isolators.

BACKGROUND OF THE INVENTION

Polarization insensitive and polarization maintaining optical isolatorsare routinely used to decouple amplifiers within laser systems and tolimit returning radiation from downstream components. Polarizationinsensitive optical isolators are typically comprised of Faraday andquartz rotator optical elements surrounded by two birefringent crystaldisplacers and a magnetic structure capable of producing a uniformmagnetic field and Faraday rotation within the Faraday optic. FIGS. 1Aand 1B demonstrate a typical fiber to fiber polarization insensitiveisolator. As shown in FIG. 1A, forward going light is separated into anextraordinary and ordinary polarized beam by the input displacer. Thesebeam polarizations are then rotated 45° clockwise by the Faraday opticand an additional 45° clockwise by the quartz rotator so that theextraordinary beam passes the output displacer without deviation whilethe ordinary beam is displaced vertically to meet it. In the reversegoing direction, as shown in FIG. 1B, the output displacer separates thelight into an ordinary and extraordinary polarized beam. These beampolarizations are then rotated 45° counter clockwise by the quartzrotator and 45° clockwise by the Faraday optic so that the ordinary beampasses the input displacer without deviation while the extraordinarybeam is displaced further. This results in about 50% of the reversegoing light being directed above and about 50% below the primary forwardgoing beam for randomly polarized reverse going laser radiation. Thislight is typically absorbed by an internal aperture component to ensureisolation of the light.

Polarization maintaining optical isolators typically replace the inputand output birefringent displacers with polarizing beam splitter (PBS)cubes that are oriented such that their transmission planes aretypically 0°, 45° or 90° apart (rotated to account for 45° Faradayrotation and any quartz rotator or half waveplate rotation desired whichis typically) 45°. In this configuration, a forward propagating,linearly polarized beam passes the input PBS cube and its polarizationis rotated 45° clockwise by the Faraday optic and, for example, anadditional 45° clockwise by the quartz rotator. The beam is thus rotatedto be aligned with the output PBS cube and passes uninhibited. In thereverse going direction, the polarized beam passes the output PBS cubeand its polarization is rotated 45° counterclockwise by the quartzrotator and 45° clockwise by the Faraday optic. After passage throughthe Faraday optic, the beam is aligned to the extinction plane of theinput PBS cube and is therefore reflected by the hypotenuse away fromthe forward propagating beam and absorbed by an internal component.

Although these optical layouts have been used in fiber lasers for manyyears at powers below 20W, current state of the art fiber laser systemsrequire operation above 40W in an ever-decreasing package size. If theclassic isolator layout as described is applied; many limitations becomeclear. To begin, the classic layout is physically very large. Assumingminimum spacing between optics and typical fiber collimator spacing, theisolator is at least 75 mm long. In addition, the surrounding magnetstructure that is required to achieve Faraday rotation is inefficientlylarge due to the layout's requirement to achieve high fields over a longFaraday optic. Furthermore, it is impossible to mount the Faraday opticwithin the classic layout in such a manner that significantly reduce itsinternal thermal gradients. These thermal gradients cause both pointingshifts of the outgoing beam as well as a reduction in beam quality.These effects are only exaggerated in high reverse power conditionswhere absorption of reverse going power causes additional heating andpotential catastrophic thermal failures.

What is needed is a high-power optical isolator which enables small sizeand limited power dependent effects.

SUMMARY OF THE INVENTION

The present invention provides a high-power optical isolator whichenables small size and limited power dependent effects. According to anaspect of the present invention, an optical isolator for use with highpower, collimated laser radiation enabling small size and limited powerdependent instability, includes an input polarizing optical element, atleast one Faraday optical element, at least two reflective opticalelements for reflecting laser radiation to provide an even number ofpasses through the at least one Faraday optical element, at least onereciprocal polarization altering optical element, an output polarizingoptical element, at least one light redirecting element for remotelydissipating isolated or lost laser radiation, and at least one magneticstructure capable of generating a uniform magnetic field within theFaraday optical element, which is generally aligned to the path of thecollimated laser radiation. A mechanical structure holds the opticalelements to provide thermal gradients that are generally aligned to thepath of the collimated laser radiation and that provide thermal andmechanical isolation between the magnetic structure and the opticalelements.

These and other objects, advantages, purposes and features of thepresent invention will become apparent upon review of the followingspecification in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic showing a forward going operation of traditionalpolarization insensitive isolator;

FIG. 1B is a schematic showing a reverse going operation of traditionalpolarization insensitive isolator;

FIG. 2A is a schematic showing a forward going operation of a preferredembodiment of the polarization insensitive configuration of the presentinvention;

FIG. 2B is a schematic showing a reverse going operation of a preferredembodiment of the polarization insensitive configuration of the presentinvention;

FIG. 3 is a schematic showing a forward going operation of an embodimentof the present invention using an output rejection mirror;

FIG. 4 is a schematic showing a coupling of isolated power into fiberoptic with dual cladding for remote dissipation of the isolated or lostlaser radiation in a cladding mode stripping splice;

FIG. 5 is a schematic showing a top view of a practical application ofthe present invention;

FIG. 6 is a side view of the practical application of FIG. 5;

FIG. 7 is a bottom view of the practical application of FIG. 5;

FIG. 8 is an end view of the practical application of FIG. 5;

FIG. 9 is an opposite end view of the practical application of FIG. 8;

FIG. 10 is a schematic showing a forward going operation of a preferredembodiment of the polarization maintaining configuration of the presentinvention showing alternate embodiment of irregular hexagon Faradayoptic;

FIG. 11 is a schematic showing a reverse going operation of a preferredembodiment of the polarization maintaining configuration of the presentinvention showing alternate embodiment of irregular hexagon Faradayoptic;

FIG. 12 is a sectional view of an optical isolator in accordance withthe present invention; and

FIG. 13 is a plan view of a Faraday optic in accordance with the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides a high-power optical isolator whichenables small size and limited power dependent effects. In a preferredembodiment of the present invention, and as shown in FIG. 2A, agenerally collimated beam of laser radiation 21 (emitted by a lasersource 20) is directed at the input face of an input displacer 24 viathe inner aperture of a rejection mirror 22. This beam is separated intoan extraordinary and ordinary polarized beam by the input displacer.These beam polarizations are then rotated 45° clockwise by passagethrough an irregular hexagon shaped Faraday optic 26 housed within amagnetic structure 28, and then rotated by an additional 45° clockwiseby a reciprocal optical rotator 30. The beams are then recombined by anoutput displacer 32 to form a single output beam of laser radiation.

With light traveling in the reverse going direction (FIG. 2B), agenerally collimated beam of laser radiation is directed at the outputface of the output displacer which separates the beam into an ordinaryand extraordinary polarized beam of laser radiation. The beampolarizations are rotated 45° counter clockwise by the reciprocaloptical rotator and then 45° clockwise by passage through an irregularhexagon shaped Faraday optic. The extraordinary beam is then displacedagain by the input displacer while the ordinary beam continues on itspath. This results in the ordinary beam being displaced below and theextraordinary beam being displaced above the forward going generallycollimated beam of laser radiation. The ordinary and extraordinary beamsare then reflected off of the rejection mirror away from the forwardgoing generally collimated beam of laser radiation, thereby isolatingthe reverse going power. In addition to isolating the reverse goingpower from the forward going power, the rejection mirror allows for thepower to be remotely dissipated which eliminates heating within theisolator.

In this preferred embodiment of the present invention, the inputdisplacer, output displacer, reciprocal optical rotator, and irregularhexagon shaped Faraday optic are all mounted to the single piece opticholder. In addition, the irregular hexagon shaped Faraday optic has atleast two surfaces which are coated with a high reflection coating toensure proper passage of the beams through the isolator. Furthermore,the irregular hexagon shaped Faraday optic is mounted to the singlepiece optic holder onto at least one of the high reflection coatedsurfaces to ensure thermal gradients that are well aligned to thegenerally collimated beams. Finally, the magnetic structure surroundingthe irregular hexagon shaped Faraday optic is thermally and mechanicallyisolated from each other by, for example, the use of a small air gap.

In an additional embodiment of the present invention, an outputrejection mirror 34 may be added to the system as shown in FIG. 3. Theforward going light passes through the center aperture of an outputrejection mirror after recombination within the output displacer. Thisis beneficial in high power situations where heating within theirregular hexagon shaped Faraday optic causes the polarization rotationwithin the Faraday optic to be not 45°. In this condition, theextraordinary and ordinary polarized beams do not properly recombinewithin the output displacer and, instead, form additional beams aboveand below the primary forward going beam. These beams will be rejectedby the output rejection mirror so that they can be properly dissipatedremotely; thereby limiting additional isolator heating and failures.

In an additional embodiment of the present invention, in fiber coupledapplications where the light is being launched from an input fiber andrecoupled into an outgoing fiber, the fiber collimating lenses can bedesigned such that the offset beams would be focused into the claddingof the dual clad fibers as shown in FIG. 4. This light would then bedissipated remotely at the cladding mode stripper which are typicallyplaced throughout the fiber laser systems. This provides for an evensmaller package size while limiting high power heating of the isolator.

In an additional embodiment of the present invention, in polarizationmaintaining applications where polarized laser radiation is beingdirected at the optical isolator, the input rejection mirror and inputdisplacer can be replaced with an input PBS cube and the outputdisplacer and optional output rejection mirror can be replaced by anoutput PBS cube. This allows for a further decrease in isolator sizewhile maintaining the high-power performance that is enabled by thepresent invention (FIGS. 5-9).

In an additional embodiment of the present invention, in broadbandapplications where dispersion within the irregular hexagon shapedFaraday optic would cause a reduction in performance, the irregularhexagon shaped Faraday optic 26 may be configured as shown in FIGS. 10and 11, for example. In this alternate configuration, the first surfaceof a terbium gallium garnet (TGG) or a potassium terbium fluoride (KTF)26 is broken into three regions. Where the outer regions are opticallycoated with an anti-reflection coating and the center region is coatedwith a high reflection coating. The three opposing surfaces are coatedwith high reflection coatings. This directs the forward and reversegoing laser radiation to pass through the Faraday optic 26 in a mannerthat would not induce deleterious dispersion related effects while stillenabling small size and high power performance.

Referring to FIG. 12, in some aspects, a high-power optical isolator 120includes a mechanical structure (i.e., an outer housing) 130. The outerhousing 130 includes an optical holder 126 that contains the displacers,rotators, etc. for directing the beam path. A Faraday optic 122 (e.g.,an irregular hexagon shaped Faraday optic such as TGG or KTF) is mountedto an end of the optical holder. The Faraday optic is surrounded by amagnetic structure 124. The optical holder 126 is coupled to the outerhousing 130 via a pliable layer 128 that mechanically isolates the inneroptical holder 126 from the outer housing 130. The pliable layer 128 mayinclude an adhesive to bond the optical holder 126 in place. The pliablelayer 128 may compress and decompress in response to both internalforces (e.g., forces generated by the magnetic structure 124) andexternal forces (e.g., forces applied to the outer housing 130 via anexternal source), thus mechanically isolating the optical holder 126(and similarly the Faraday optic 122) from the outer housing 130. Insome aspects, the pliable layer 128 is thermally conductive or thermallyisolating in order to thermally isolate the optical holder 126 from theouter housing 130. Optionally, a heatsink may be mounted to the outerhousing 130 to dissipate any heat absorbed by the isolator 120. An airgap 132 that surrounds the Faraday optic 122 and separates the Faradayoptic 122 from the magnetic structure 124 (and the outer housing 130)may provide further thermal and mechanical isolation for the Faradayoptic 122.

Referring now to FIG. 13, the Faraday optic 26 may be an irregularhexagon shape with a front surface 26 a that is parallel to an oppositesurface 26 d of the Faraday optic 26. The Faraday optic 26 may bemounted such that the beam of laser radiation enters the Faraday optic26 perpendicular to the front surface 26 a. The irregular hexagon shapedFaraday optic 26 may also include two or more front angled surfaces 26 bthat are angled relative to the front surface 26 a by angle 27. That is,each angled surface 26 b may have the same angle 26 relative to thefront surface 26 a. The angle is greater than 2 degrees. Beams of laserradiation that enter the Faraday optic 26 at the front angled surface 26b may have an external angle of incidence a and an internal angle ofincidence β. Beams of laser radiation may enter the Faraday optic 26 andreflect off of the opposite surface 26 d with a reflection angle θ. TheFaraday optic 26 may have a length L that corresponds to a distancebetween the front surface 26 a and the opposite surface 26 d. When theindex of refraction of air is represented by n₁ and the index ofrefraction of the Faraday optic 26 is represented by n₂, the externalangle of incidence α, the external angle of incidence and the reflectionangle θ have a relationship as shown in Equations (1) and (2).

n₁ sin α=n₂ sin β  (1)

θ=2(α−β)   (2)

The length L of the Faraday optic 26 and an approximately three timesthe diameter of the beam of laser radiation D (i.e., D≅3* beam diameter)have a relationship as shown in Equation (3).

$\begin{matrix}{{\tan\left( {0.5\theta} \right)} = \frac{0.5D}{L}} & (3)\end{matrix}$

Thus, when taking into account the appropriate refractive indexes, anappropriate length L of the Faraday optic may be selected for a givenbeam diameter. For example, for a 0.5 millimeter diameter beam (i.e., Dis 1.5) the Faraday optical may have a length L of 8 mm, 0 may be 10.71degrees, a may be 10.91 degrees, and may be 5.56 degrees.

The irregular hexagon shaped Faraday optic 26, when mounted to theoptical holder 126 by front surface 26 a (or, in some examples, by theopposite surface 26 d (FIGS. 10 and 11)), thermally aligns temperaturegradients through the Faraday optic 26. Thus, beams that pass throughthe Faraday optic on the same plane experience the same temperaturegradients. This ensures that the thermal gradients do not cause beampointing (i.e., drifting of the beam position from the ideal position).That is, aligning the temperature gradients increases beam pointstability.

The present invention is thus an improvement over the prior art andprovides for a high-power polarization insensitive and polarizationmaintaining optical isolator enabling small size and limited powerdependent effects. In contrast to the prior art, the irregular hexagonshaped Faraday optic with at least two high reflection coatings allowthe input displacer, output displacer, and reciprocal rotator to beplaced adjacent to each other, thereby reducing the isolators overalllength. In addition, by mounting the irregular hexagon shaped Faradayoptic such that any absorbed power which heats the Faraday optic willproduce thermal gradients well aligned to the generally collimated beamsensures limited power dependent thermal pointing shifts. The additionalbenefit of this mounting configuration is that a long optical pathlengthwithin the Faraday optic is not deleterious to the isolator'sperformance. This permits the use of a long optical pathlength withinthe Faraday optic while enabling a highly efficient and minimal volumeof magnets to produce 45° of Faraday rotation. Furthermore, by thermallyisolating the magnet structure from the optic structure, the absorbedpower within the optics only heats the optic structure and not themagnets; thereby reducing the power dependent transmission loss due toheating by as much as 25%. Finally, by remotely dissipating the isolatedreverse going power and lost forward going power the present inventioneliminates heating of the isolator which would cause significant powerdependent effects.

Changes and modifications in the specifically described embodiments canbe carried out without departing from the principles of the invention,which is intended to be limited only by the scope of the appendedclaims, as interpreted according to the principles of patent lawincluding the doctrine of equivalents.

1. An optical isolator, comprising: a Faraday optic havingopposite-facing front and back sides, the back side having a planarsurface, the front side having: a first planar surface parallel with theplanar surface of the back side, and second and third planar surfacesarranged such that the first planar surface is between the second andthird planar surfaces, each of the second and third planar surfacesbeing angled away from the first planar surface at an oblique anglerelative to the first planar surface; and high-reflection coatings onthe planar surface of the back side and the first planar surface of thefront side to provide an even number of passes between the front andback sides of forward-going laser radiation that is incident on theFaraday optic at the second planar surface at an initial propagationdirection, the oblique angles of the second and third planar surfaces ofthe front side causing the forward-going laser radiation to leave theFaraday optic through the third planar surface at a subsequentpropagation direction that is opposite and offset from the initialpropagation direction.
 2. The optical isolator of claim 1, wherein theoblique angle is greater than two degrees.
 3. The optical isolator ofclaim 1, further comprising: an input polarizing optical elementconfigured to separate two orthogonally polarized components ofcollimated laser radiation and forward at least one of the twoorthogonally polarized components along the initial propagationdirection to the second planar surface of the front side of the Faradayoptic as the forward-going laser radiation; and an output polarizingoptical element arranged to intercept the forward-going laser radiationwhen propagating away from the third planar surface of the front side ofthe Faraday optic after the even number of passes; a reciprocalpolarization altering optical element arranged to intercept theforward-going laser radiation between the input and output polarizingoptical elements; and a light redirecting element arranged to remotelydissipate isolated reverse-going laser radiation.
 4. The opticalisolator of claim 3, further comprising a magnetic structure forgenerating a magnetic field within the Faraday optic, the magneticstructure surrounding but being separated from the Faraday optic.
 5. Theoptical isolator of claim 4, wherein the Faraday optic and the magneticstructure are configured to rotate polarization of forward-going laserradiation by a total of 45 degrees in a first rotation direction; andthe reciprocal polarization altering optical element is configured torotate polarization of the forward-going laser radiation by anadditional 45 degrees in the first rotation direction.
 6. The opticalisolator of claim 3, wherein: the input polarizing optical element is afirst birefringent displacer configured to forward both of the twoorthogonally polarized components as the forward-going laser radiation;and the output polarizing optical element is a second birefringentdisplacer configured to combine the two orthogonally polarizedcomponents into a single output beam.
 7. The optical isolator of claim6, wherein the light redirecting element is a rejection mirror disposedin an input propagation path of the collimated laser radiation towardthe first birefringent displacer, the rejection mirror being arranged todeflect reverse-going laser radiation displaced from the inputpropagation path.
 8. The optical isolator of claim 6, further comprisinga rejection mirror disposed in an output propagation path of the singleoutput beam, the rejection mirror being arranged to deflectforward-going laser radiation not combined by the second birefringentdisplacer.
 9. The optical isolator of claim 6, further comprising: anoptical fiber for receiving and transmitting the single output beam; anda lens for coupling the single output beam into the optical fiber anddirecting, into a cladding of the optical fiber, forward-going laserradiation not combined by the output polarizing optical element.
 10. Theoptical isolator of claim 3, wherein: the input polarizing opticalelement is a first polarizing beam splitter cube configured to transmitonly a selected one of the two orthogonally polarized components to theFaraday optic; and the output polarizing optical element is a secondpolarizing beam splitter cube.
 11. The optical isolator of claim 3,wherein the reciprocal polarization altering optical element is ahalf-wave plate.
 12. The optical isolator of claim 1, wherein theoptical isolator is configured for fiber coupling to an input fiber andan outgoing fiber.
 13. The optical isolator of claim 1, wherein theFaraday optic includes potassium terbium fluoride.
 14. An opticalisolator, comprising: a Faraday optic having opposite-facing front andback sides, the front side having a planar surface including first andsecond outer regions and a center region disposed between the first andsecond outer regions, the back side having: a first planar surfaceparallel with the planar surface of the front side, and second and thirdplanar surfaces arranged such that the first planar surface is betweenthe second and third planar surfaces, each of the second and thirdplanar surfaces being angled away from the first planar surface at anoblique angle relative to the first planar surface; and high-reflectioncoatings on the center region of the planar surface of the front sideand the first, second, and third planar surface of the back side toprovide an even number of passes between the front and back sides offorward-going laser radiation that is incident on the front side of theFaraday optic at the first outer region at an initial propagationdirection, the oblique angles of the first and second planar surfaces ofthe back side causing the forward-going laser radiation to leave theFaraday optic through the second outer region of the planar surface ofthe front side at a subsequent propagation direction that is oppositeand offset from the initial propagation direction.
 15. The opticalisolator of claim 14, further comprising: an input polarizing opticalelement configured to separate two orthogonally polarized components ofcollimated laser radiation and forward at least one of the twoorthogonally polarized components along the initial propagationdirection to the first outer region of the planar surface of the frontside of the Faraday optic as the forward-going laser radiation; and anoutput polarizing optical element arranged to intercept theforward-going laser radiation when propagating away from the secondouter region of the planar surface of the front side of the Faradayoptic after the even number of passes; a reciprocal polarizationaltering optical element arranged to intercept the forward-going laserradiation between the input and output polarizing optical elements; anda light redirecting element arranged to remotely dissipate isolatedreverse-going laser radiation.
 16. The optical isolator of claim 15,further comprising a magnetic structure for generating a magnetic fieldwithin the Faraday optic, the magnetic structure surrounding but beingseparated from the Faraday optic, the Faraday optic and the magneticstructure being configured to rotate polarization of forward-going laserradiation by a total of 45 degrees in a first rotation direction, thereciprocal polarization altering optical element being configured torotate polarization of the forward-going laser radiation by anadditional 45 degrees in the first rotation direction.
 17. The opticalisolator of claim 15, wherein: the input polarizing optical element is afirst birefringent displacer configured to forward both of the twoorthogonally polarized components as the forward-going laser radiation;and the output polarizing optical element is a second birefringentdisplacer configured to combine the two orthogonally polarizedcomponents into a single output beam.
 18. The optical isolator of claim17, wherein the light redirecting element is a rejection mirror disposedin an input propagation path of the collimated laser radiation towardthe first birefringent displacer, the rejection mirror being arranged todeflect reverse-going laser radiation displaced from the inputpropagation path.
 19. The optical isolator of claim 17, furthercomprising a rejection mirror disposed in an output propagation path ofthe single output beam, the rejection mirror being arranged to deflectforward-going laser radiation not combined by the second birefringentdisplacer.
 20. The optical isolator of claim 15, wherein: the inputpolarizing optical element is a first polarizing beam splitter cubeconfigured to transmit only a selected one of the two orthogonallypolarized components to the Faraday optic; and the output polarizingoptical element is a second polarizing beam splitter cube.